Upscaling environment-friendly cavitation melt treatment (UltraMelt #2)

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Our use of metals is so important that it defines periods of human civilisation - from the Bronze Age (c. 3600 BC) to the Iron Age (c. 1100BC). With our present-day mastery of metals and alloys, the mounting emphasis is now on resources and the environment. The metals industry is looking at new ways to produce lighter, stronger materials in a sustainable, economical and pollution-free manner. Ultrasonic cavitation treatment offers a route to meet these goals. Ultrasonic treatment of the second commonest structural metal, aluminium, causes degassing through the evacuation of dissolved gases that lead to porosity, grain refinement to assist formability, dispersion and distribution of solid or immiscible phases to improve mechanical properties during recycling etc. In spite of the benefits, transfer of this promising technology to industry has been plagued by difficulties, especially in treating large volumes of liquid metal typical in processes such as 'Direct Chill' continuous casting for ingot production. Fundamental research is needed to answer the following practical questions: what is the optimum melt flow rate that maximises treatment efficiency whilst minimizing input power, cost, and plant complexity? What is the optimum operating frequency and acoustic power that accelerates the treatment effects? What is the optimum location of an ultrasonic power source in the melt transfer system in relation to the melt pool geometry? Answering these questions will pave the way for widespread industrial use of ultrasonic melt processing with the benefit of improving the properties of lightweight structural alloys, simultaneously alleviating the present use of polluting (Cl, F) for degassing or expensive (Zr, Ti, B, Ar) grain refinement additives.Capitalising on the unique expertise gained by the proposers during the highly successful UltraMelt project (22 publications), this research aims to answer the challenge of efficiently treating large liquid volumes by developing a comprehensive numerical model that couples all the physics involved: fluid flow, heat transfer, solidification, acoustics and bubble dynamics. Greenwich will lead the development of an improved cavitation model, based on the wave equation and conservation laws, and applied to the two-phase problem of bubble breakup and transport in the melt, and its interaction with solid inclusions (e.g. the solidification front of an aluminium alloy or of any intermetallic impurities present). To improve the efficiency of the ultrasonic cavitation treatment in flowing metal, a launder conduit will be used. The sensitivity of the process with respect to different adjustable parameters (source power, frequency, time in the cavitation zone, baffle location ...) will be examined with parallel computations in a 3D model of melt flow in the launder. This computer model will be validated by experiments in both transparent liquids and aluminium. Water and transparent organic alloy experiments will use a PIV technique by Oxford Brookes University to measure the size, number and positions of bubbles and compared these with the numerical predictions. Mechanisms of intermetallic fragmentation and particle cluster breakup will be observed in real time using a high speed camera at Brunel University and X-ray radiography at the Diamond Light Source facility. Mechanical properties of intermetallic impurities at temperatures relevant to melt processing will be measured using unique nano-indentation technique in collaboration with Anton Paar Ltd. Cavitation pressure measurements in launder conduits will be conducted at Brunel University and the empirical observations will be compared with model predictions. The fully-developed model will be used to optimise the ultrasonic melt treatment in melt flow during direct-chill casting and verified using pilot-scale facilities at AMCC (Brunel, with support of Constellium) and industrial-scale facilities at Kaiser Aluminum.